Combination of being so gifted and being underchallenged spells trouble. (Perfectionism, among other things, and, as a consequence, everything for him either being 'too hard' or 'too easy/boring', is most common.)
His math is supposed to be his strength but now he seemed to lose interest in it and complained that it is hard, mind you it’s only 1 grade ahead.
The material could be just boring - plus he could be 'burned out' in general from boredom at school. (And most of math material I've seen, including well-know major programs, with very few exceptions, IS boring; 'Everyday Math' program is in addition damaging.)
Try offering 'non-standard' materials to him (*instead* of forced standard learning, at least for now, until he 'decompresses'). The following recommendations may be ahead of his chronological age, but see if he gets hooked on any of these. Most are available from the library or for free online. You could start with David Macaulay's books (they are awesome, with one-of-a kind illustrations) + "Great Source" math books + "George's ..." books.
- "The new way thing work" by David Macaulay - physics/mechanics/technology
- "The way we work" by same author - human biology
- "Asimov on chemistry" by (one and only) Isaac Asimov
- "The disappearing spoon" - chemistry
- "George's secret key to the universe", etc. by Lucy and Stephen Hawkin - physics/astronomy
- any astronomy books
- "For the love of physics" by prof. Lewin (of MIT)
- "Understanding thermodynamics" by H.C. Van Ness - physics (this may sound very advanced, but the presentation starts with 'sugar cubes being thrown out of the window', which is quite accessible)
- "Trouble with physics" by Lee Smolin
- "Great Source" reference books: "Math to know/at hand/on call" - "Algebra/geometry to go" etc.
- Nova science videos online
- "Particle physics for non-physicists" videos online
The above would work for a 'sciency' child. Other people here may suggest something for humanities.
If he likes words:
- vocabulary.com (create a (free) account for him - it is like a game).
ETA: Also what MON said:
learned concepts when she needed them and really didn't learn well when she was learning procedures
."Great Source" math books are good in this respect because they are "reference" books (and also well illustrated) - the child may run as far ahead as he wants to and pick and choose.
Another similar book is "Math Dictionary for Kids: The Essential Guide to Math Terms, Strategies, and Tables"
by Theresa Fitzgerald. (This one is quite eclectic, AFAIR.)